In a typical consumable electrode arc welding, welding is performed by feeding a welding wire as a consumable electrode at a constant feeding rate and generating an arc between the welding wire and base material. In the consumable electrode arc welding, both the welding wire and the base material are mostly placed in a welding state in which a short circuit state and an arc generation state are alternately repeated. In a small current region where an average welding current value is less than about 180 A (a feeding rate is 4 m/min), short circuit and arc is repeated with an almost constant cycle. Thus in the small current region, the welding can be performed with a small generation amount of spatter and good bead appearance by suitably controlling the welding current and a welding voltage.
in contrast, in a large current region where the average welding current value is about 180 A or more, the cycle of short circuit and arc is longer than a suitable value and a variance of the cycle is large. Thus in the large current region, the generation amount of spatter tends to increase and the bead appearance tends to degrade even if the welding current and the welding voltage are controlled accurately. In order to improve this, there has been proposed a welding method of alternating feeding of the welding wire between forward feeding and reverse feeding cyclically (see a patent document 1, for example). Hereinafter this welding method will be explained.
FIG. 8 is a waveform diagram of the welding method in which the forward feeding and the reverse feeding of the feeding rate is repeated cyclically. (A) of this figure shows a waveform of a feeding rate Fw, (B) of this figure shows a waveform of a welding current Iw and (C) of this figure shows a waveform of a welding voltage Vw. Hereinafter explanation will be made with reference to this figure.
As shown in (A) of this figure, in the feeding rate Fw, an upper side and a lower side than 0 represent a forward feeding period and a reverse feeding period, respectively. The forward feeding represents feeding of the welding wire in a direction approaching the base material, whilst the reverse feeding represents feeding of the welding wire in a direction separating from the base material. The feeding rate Fw has a waveform which changes sinusoid ally and shifts on the forward feeding side. Thus as an average value of the feeding rate Fw is positive, the welding wire is fed forwardly in average.
As shown in (A) of this figure, the feeding Fw is 0 at a time t1, in a forward feeding acceleration period during a period from the time t1 to a time t2, the maximum value of the forward feeding at the time t2, in a forward feeding deceleration period during a period from the time t2 to a time t3, 0 at the time t3, in a reverse feeding acceleration period during a period from the time t3 to a time t4, the maximum value of the reverse feeding at the time t4, and in a reverse feeding deceleration period during a period from the time t4 to a time t5.
Short circuit between the welding wire and the base material occurs mostly before or after the maximum value of the forward feeding at the time t2. This figure shows a case where the short circuit occurs at a time t21 during the forward feeding deceleration period after the maximum value of the forward feeding. If the short circuit occurs at the time t21, the welding voltage Vw rapidly reduces to a short-circuit voltage value of a few volts as shown in (C) of this figure, and the welding current Iw also reduces to an initial current value of a small current value as shown in (B) of this figure. Thereafter the welding current Iw increases with a predetermined inclination. When the welding current reaches a predetermined peak value, the welding current is kept at this value.
As shown in (A) of this figure, from the time t3, as the feeding rate Fw is placed in the reverse feeding period, the welding wire is reversely fed. The short circuit is released by this reverse feeding, and hence an arc is regenerated at a time t31. The arc is regenerated mostly before or after the maximum value of the reverse feeding at the time t4. This figure shows a case where the arc is generated at the time t31 during the reverse feeding acceleration period before the peak value of the reverse feeding.
If the arc is regenerated at the time t31, the welding voltage Vw increases rapidly to an arc voltage value of several ten volts as shown in (C) of this figure. As shown in (B) of this figure, according to a narrow-part detection control of a droplet for detecting a sign of arc regeneration, the welding current Iw rapidly reduces from a time slightly before the time t31 and becomes a small current value at the arc regeneration time of t31.
As shown in (A) of this figure, from the time t31 to the time t5, the feeding rate Fw is placed in a reverse feeding state. An arc length becomes long during this period. As shown in (B) of this figure, during the period from the time t31 to the time t5, the welding current Iw increases with a predetermined inclination, then reaches a predetermined high arc current value and maintains this value for a predetermined period, and thereafter starts reducing.
As shown in (A) of this figure, the feeding rate Fw is placed in the forward feeding period from the time t5 and reaches a forward feeding peak value at a time t6. Then short circuit occurs at a time t61. During a period from the time t5 to the time t61, the welding voltage Vw reduces gradually as shown in (C) of this figure, and the welding current Iw also reduces gradually as shown in (B) of this figure.
As described above, the cycle of short circuit and arc substantially coincides with the cycle of forward feeding and reverse feeding as to the feeding rate. That is, according to this welding method, the cycle of short circuit and arc can be set to a desired value by suitably setting the cycle of forward feeding and reverse feeding as to the feeding rate. Thus, in particular, if this welding method is implemented in the large current region, the cycle of short circuit and arc can be suppressed in its variance and made substantially constant. Consequently the welding can be performed with a small generation amount of the spatter and the good bead appearance.
However, in the welding method of repeating the forward feeding and the reverse feeding as to the feeding rate, there arises a case where the short circuit does not occur at the suitable timing due to disturbance such as irregular movements of a molten pool and a distance between a power supply tip and the base material, and a change of a welding position. In this case, as the cycle of short circuit and arc does not synchronise with the cycle of forward feeding and reverse feeding, the cycle of short circuit and arc varies. A method of restoring this asynchronous state to a synchronous state is disclosed in a patent document 1.
According to an invention of the patent document 1, in a case where short circuit does not occur until a feeding rate reaches a predetermined feeding rate during feeding-rate deceleration in the forward feeding of a welding wire, cyclical change of the feeding rate is stopped and the feeding rate is controlled to a constant value of a first feeding rate. When the short circuit occurs in the forward feeding at the first feeding rate, reduction of the feeding rate from the first feeding rate is started and cyclical change of the feeding rate is restarted to perform the welding. Consequently, the asynchronous state is intended to be restored to the synchronous state.